Revealing the aging process of solid electrolyte interphase on SiOx anode

As one of the most promising alternatives to graphite negative electrodes, silicon oxide (SiOx) has been hindered by its fast capacity fading. Solid electrolyte interphase (SEI) aging on silicon SiOx has been recognized as the most critical yet least understood facet. Herein, leveraging 3D focused ion beam-scanning electron microscopy (FIB-SEM) tomographic imaging, we reveal an exceptionally characteristic SEI microstructure with an incompact inner region and a dense outer region, which overturns the prevailing belief that SEIs are homogeneous structure and reveals the SEI evolution process. Through combining nanoprobe and electron energy loss spectroscopy (EELS), it is also discovered that the electronic conductivity of thick SEI relies on the percolation network within composed of conductive agents (e.g., carbon black particles), which are embedded into the SEI upon its growth. Therefore, the free growth of SEI will gradually attenuate this electron percolation network, thereby causing capacity decay of SiOx. Based on these findings, a proof-of-concept strategy is adopted to mechanically restrict the SEI growth via applying a confining layer on top of the electrode. Through shedding light on the fundamental understanding of SEI aging for SiOx anodes, this work could potentially inspire viable improving strategies in the future.

2. The authors are suggested to pay more attention to check the full manuscript, since some errors are present.For example, the authors mentioned Fig. 2j k in the manuscript, but there is no corresponding Fig. 2k in Fig. 2 at all.In addition, the description of Fig. S8 is divided into Fig.S8a,b,d and d, but Fig. 8 is not marked with a-d diagram.G 3. In page 6, the authors mentioned that "After 50 cycles, the thickness of SEI layer increased from 100 nm to approximately 250 nm", but no description of "100 nm" could be found in the previous paragraphs.
4. The authors believed that the conductivity of SEI comes from the carbon black particles doped by outward expansion during SEI aging, but why the trend of SEI resistance measured in Fig. 4d is consistent with SiOx.
5. The authors are suggested to confirm whether Fig. S7d is the enlarged area corresponding to Fig. S7c, and if so, the corresponding area should be marked in Fig. S7c.
6.The authors declared that the aging and thickening process of SEI gradually increases from inside to outside, and gradually densifies, then should the outermost SEI be the most stable?However, in the description of Fig. S9, the authors declared that "SEI is not completely stable even after a long cycle".Is this contradictory?7. The authors declared that "After the first lithiation, a porous SEI layer could be observed around the particle.Surprisingly, this SEI layer dramatically diminished after the first delithiation, which is supported by the XPS results (Supplementary Fig. 8)."I want to know what's the reason for no C1s peak could be found in the 1st lithiation, 300th lithiation/delithiation.
Reviewer #2 (Remarks to the Author): Qian et al. performed a very detailed study of the microstructural behavior of a SiOx negative electrode material through very high level characterization techniques.A considerable and delicate work was performed on a very important matter.In particular FIB sectioning-3D reconstruction as well as thin lamella for TEM and STEM-EELS were employed to give an image of the SEI at a nanometer level.Some of the experiments are very innovative (such the conductivity measurement at the nanoscale) and deserve high attention.They developed an interesting concept of typeI and type II SEI.
Unfortunately, some of these experiments, decisive to the study of the authors, did not convince me in relation to preservation of the real sample supporting the conclusions.Considering the work to be further performed to accomplish that, I do not support publication in Nature communications.Indeed, in the case of the characterization of SEIs, due to its particularly sensitivity to interacting beams (photons, electrons, ions…), a special attention must be paid to the dose and the experimental conditions.I do not find enough of such concern in the work and I give many examples in the following to justify my opinion.
In Fig2, a "type I SEI" is proposed.But Ga+ ion, because of particular milling condition of porous (inhomogeneous) materials often amorphize/make porous surface of samples.I have seen it on samples not even containing Silicon.Based on this, I find the Fig3 a bit overstating 2 types of SEI.
In XPS spectra, the Li peak would be worth showing.But on these composite electrodes, one can always wonder what is the use of XPS when the beam is roughly 300 mx500 m.Hence, the variations observed in Supp Fig9 are rather uncertain.
XPS without argon clusters (Ar+ used in the study) would probably damage a lot the SEI and make the results mostly irrelevant to the real chemical state of the surface.
Polishing with ion-beam milling usually provokes sensible local increase of temperature so no idea if the contrast seen on some images is real (next to SiOx particles).(for example Supplementary Fig 6).
The TEM lamella, probably already modified by the FIB preparation, is surely very beam sensitive and I doubt high losses spectra can be obtained without damaging the sample containing Li2O, LiF… It would be interesting to see an image of the area after the analysis… How was transferred the TEM lamella from the FIB to the TEM?

Other comments:
No comparison with other formulation of electrodes demonstrating a worse cycling when less CA in the binder is used.The quantity of it is quite high in mass, so volume wise, there is basically acetylene black everywhere.
Washing electrodes with DMC is always a risky task: not reproducible, chemically selectiveness (soluble species) … The embedding in a resin is a typical preparation technique for various samples (Suppl Fig 10).However, these chemical compounds are not inert ones (by definition, they need to be polymerized).Consequently, I am very doubtful on the preservation of the "real SEI" in these measurements (even when delithiated…).
In Suppl.Fig 12, the "SEI" part looks more like a hole due to beam damage than a solid (porous) phase.Hence, I do not find the electric conductivity measurement on this part convincing enough.
The various EELS spectra shown in Suppl.Fig 13 are diverse and a simple extraction of CA in them to get Fig4h, is highly speculative, at least without further detail.Furthermore, Acetylene black is a CA with a very high-level sp2 carbon and EELS K-edge spectra usually present a much sharper * peak, so I find that the results do not demonstrate the clear presence of the CA in the SEI.Maybe high resolution TEM (corrected) images could provide such information.I find the "confining" idea of the SEI not clear at all, at least in its set-up and effectiveness: what about the pressure applied on the whole cell?What about the movement of the graphite "layer" along cycling (they could change quite substantially the electronic conductivity after moving inward within the electrode)?Furthermore, apart from the first cycle, subsequent ones have a smaller swelling ratio variation in the case of pure SiOx… When swelling is concerned in electrodes, a parameter to control is usually the porosity of the electrode.Nothing is said about that in the manuscript.

Other remarks:
The 2kV beam current was surely not 180 μA!I do not see the 1 micro Pt coating for example in Fig 2 .How was it deposited?
There is no Fig 2k.
The least to do is to give a value for x in SiOx.One could wonder if similar results are obtained for different values of x and it is necessary to be able to reproduce exposed results.

Reviewer #1
General Comments: In this paper, the authors first revealed an exceptionally characteristic SEI microstructure with a nanoporous inner region and a dense outer region, then discovered that the electronic conductivity of thick SEI relies on the percolation network within composed of conductive agents.Finally, the authors adopted a proof-of-concept strategy to mechanically restrict the SEI growth via applying a graphite layer on top of the electrode.I recommend the acceptance of this manuscript after the following issues are well addressed Response: We are truly thankful to the reviewer for the positive assessment of our work, and for providing valuable and insightful comments that helped us to reflect on the scientific significance of our study.We sincerely hope that this revision relieves the reviewer's concerns.

Comment #1: In introduction, the authors mentioned "to reveal the forming process of SEI on
SiOx materials, new approaches should be adopted", but they didn't indicate in what specific ways the existing testing techniques do not meet the needs of the current work.

Response:
We thank the reviewer for the constructive comment.Indeed, we should have specified the shortage of the existing characterization techniques.To better reflect the novelty and significance of our work, following revisions have been made to the manuscript: Comment #2: The authors are suggested to pay more attention to check the full manuscript, since some errors are present.For example, the authors mentioned Fig. 2j k in the manuscript, but there is no corresponding Fig. 2k in Fig. 2 at all.In addition, the description of Fig. S8 is divided into Fig. S8a,b,d and d,but Fig. 8 is not marked with a-d diagram.
Response: We apologize for these mistakes.After carefully checking all figure numbers, corrections have been made accordingly in the manuscript:

Main manuscript:
However, these above-mentioned techniques are unsuitable for directly observation of the structural evolution of SEI on micro-sized particles with large volume swings, while traditional microscope-based technique is only suitable for 2D observation of the cross-sectional structure of the electrode without 3D information.In this case, new approaches should be adopted.

Main manuscript:
Surprisingly, this SEI layer dramatically diminished after the first delithiation, which is supported by the XPS results (Supplementary Fig. 7b and 7c).
A key finding from Fig. 2d and 2e is that, after long-term cycling… Supplementary Information: Supplementary Fig. 7 | Full XPS spectra of the SiOx electrode a, before cycling; b, after 1 st lithiation; c, 1 st delithiation; d, 300 th lithiation and e, 300 th delithiation.
As shown in Supplementary Fig. 7a, C, Si, O and F element could be observed in SiOx electrode before cycling.However, the peak related to Si disappeared and the peaks for O and F element became more distinct after the initial discharging, meaning that the formation of SEI on the surface.Interestingly, as shown in Supplementary Fig. 7b and 7c, a weak peak for Si element could be found in the specimen at the 1 st delithiation, implying the SEI became thinner after the initial charging process.This result can be further verified by the weakened C-C peak (corresponding to conductive carbon) after 1 st lithiation and the intensified C-C peak after 1 st delithiation.The Si peak could not be found in the 300 th delithiated and lithiated specimens, demonstrating the thick SEI on the surface of SiOx anode after long-term cycles.
Again, we would like to thank the reviewer as this insightful comment has inspired us to better demonstrate our claims.Relevant discussions have been added accordingly:

Main manuscript:
An SEI layer is initially formed on the expanded SiOx upon the first lithiation, which is supported by the diminished C 1s and Si 2p XPS signal (Supplementary Fig. 7).During the first delithiation, the freshly formed Type I SEI cannot shrink at the same pace as the particle due to the lack of adhesion force between them, leading to partial detachment from SiOx particles (Fig. 3a), which is supported by the reappeared C 1s and Si 2p peaks (Supplementary Fig. 7) as well as the gradual increase of coulombic efficiency during initial 5 cycles (Supplementary Fig. 11).

Supplementary Information:
Interestingly, as shown in Fig. 7b and 7c, a weak peak for Si element could be found in the specimen at the 1 st delithiation, implying the SEI became thinner after the initial charging process.This result can be further verified by the weakened C-C peak (corresponding to conductive carbon) after 1 st lithiation and the intensified C-C peak after 1 st delithiation.The Si peak could not be found in the 300 th delithiated and lithiated specimens, demonstrating the thick SEI on the surface of SiOx anode after long-term cycles.

Reviewer #2
General Comments: Qian et al. performed a very detailed study of the microstructural behavior of a SiOx negative electrode material through very high level characterization techniques.A considerable and delicate work was performed on a very important matter.In particular FIB sectioning-3D reconstruction as well as thin lamella for TEM and STEM-EELS were employed to give an image of the SEI at a nanometer level.Some of the experiments are very innovative (such the conductivity measurement at the nanoscale) and deserve high attention.They developed an interesting concept of type I and type II SEI.Unfortunately, some of these experiments, decisive to the study of the authors, did not convince me in relation to preservation of the real sample supporting the conclusions.Considering the work to be further performed to accomplish that, I do not support publication in Nature communications.Indeed, in the case of the characterization of SEIs, due to its particularly sensitivity to interacting beams (photons, electrons, ions…), a special attention must be paid to the dose and the experimental conditions.I do not find enough of such concern in the work and I give many examples in the following to justify my opinion.

Response:
We sincerely appreciate the time and effort taken in reviewing our manuscript and the constructive comments provided.We tried our best to address the reviewer's concerns about the sample preservation during preparation and characterization, and carefully revised the manuscript accordingly.Also, we hope the reviewer to kindly consider that the observation of SEI on SiOx might be different to other anode materials (e.g.graphite): its much thicker (under submicron scale) and resilient against beam damage.We are truly thankful for the constructive comments that would significantly improve this work and we also hope that this revision relieves the reviewer's concerns.
Comment #1: In Fig2, a "type I SEI" is proposed.But Ga + ion, because of particular milling condition of porous (inhomogeneous) materials often amorphize/make porous surface of samples.

I have seen it on samples not even containing Silicon. Based on this, I find the Fig3 a bit overstating 2 types of SEI.
Response: We thank the reviewer for the insightful comments.We do understand your concerns regarding to the beam damage to the sample.Admittedly, we cannot guarantee perfect sample preservation since FIB is indeed a constructive sample preparation method.However, whether the beam damage will interfere our observation depends on how the sensitivity of sample to beam and the scale of observation.In order to verify the suitability of our preparation method, we have also prepared sample through cryo-ultramicrotomy, where samples were sliced up mechanically under cryogenic environment.This is one of the best methods we can think of to reveal the crosssectional structure of SiOx anodes with minimum sample damage.It should be noted that being unable to connected with an imaging system (e.g.SEM), cryo-ultramicrotome was not adopted in this work since we aim to obtain a 3D reconstructed graph.As a result, very similar SEI structures can be observed.From this comparison, we can conclude that beam damage will not affect SEI morphology at sub-micron scales.

Manuscript:
Additionally, to preclude the interference of beam damage to the sample, cryo-ultramicrotomy was also carried out to prepare the sample.As shown in Supplementary Fig. 10, both methods result in similar SEI morphology, suggesting beam damage is negligible in this study.
Regarding to the reviewer's concern that we might have overstated two types of SEI, we would like to point out that we did not mean to define them as two different types of SEI layer.An earlystage SEI (Type I) generally show a porous structure, during repeated cycling, it will develop into a dense SEI (Type II).We agree that there should not be a clear line to draw them apart as they might be similar in terms of chemical compositions.We used such terminology only to emphasis the process of SEI densification, to avoid unnecessary confusion, additional discussions are added to the revised manuscript: Comment #2: In XPS spectra, the Li peak would be worth showing.But on these composite electrodes, one can always wonder what is the use of XPS when the beam is roughly 300 um×500 um.Hence, the variations observed in Supp Fig9 are rather uncertain.

Response:
We thank the reviewer for the comments.We agree that the small variation in XPS might not be statically significant.Considering that the chemical composition of SEI is quite irrelevant and the information does not help us to better demonstrate the main claims of this work, we have decided to drop Supplementary Fig. 9 from this work to void unnecessary distraction.

Comment #3: XPS without argon clusters (Ar + used in the study) would probably damage a lot
the SEI and make the results mostly irrelevant to the real chemical state of the surface.

Response:
We thank the reviewer for the comments.We agree that Ar + etching could potentially change the chemical state of SEI.However, if we are just looking at the full spectra, we can see that Ar + etching barely makes any difference as the etching process will only cause slight chemical shift.
In this work, Ar + was adopted because a thick SEI on the very top of the electrode (not necessarily on SiOx) has prevented us to observe the SEI on SiOx.For instance, the difference of Si signal between 1 st lithiation and delithiation becomes more distinctive without Ar + etching:

Manuscript:
Note that Type I and Type II SEI only differ in the morphology while they might exhibit very similar chemical compositions.

Comment #4: Polishing with ion-beam milling usually provokes sensible local increase of temperature so no idea if the contrast seen on some images is real (next to SiOx particles). (for example Supplementary Fig 6).
Response: We thank the reviewer for the comments.Indeed, after reconsideration, we find the conclusion drawn from Supplementary Fig. 6 in the original manuscript, which is an optical photo, might be far-fetched.Therefore, we have dropped this figure and deleted relevant discussions: The corresponding optical images are shown in Supplementary Fig. 6.It can be observed that the pristine particles are covered by a matrix of carbon black and polymer binder, which serves as the electron percolation network of active materials.
As we have previously demonstrated in the response to Comment #1, FIB and ion-milling will not affect the structure of SEI in SEM images.3X Ar-ion beam milling will cause the sample temperature to increase to approximately 70 ℃, which should not cause significant damage to the main components of SEI.Moreover, even if ion-milling somehow caused the discoloration of SEI under optical microscope, it would not affect any conclusion we have drawn.Instead, such contrast helps us to confirm that the region around SiOx is indeed SEI by cross-referencing the SEM and the optical images (Supplementary Fig. 8d and 8e in the revised manuscript).

Comment #5:
The TEM lamella, probably already modified by the FIB preparation, is surely very beam sensitive and I doubt high losses spectra can be obtained without damaging the sample containing Li2O, LiF… It would be interesting to see an image of the area after the analysis… How was transferred the TEM lamella from the FIB to the TEM?
Response: We thank the reviewer for the detailed comments.We understand the concerns from the reviewer, but these factors will not affect our conclusion for two reasons: (1) Map scanning mode was adopted in the EELS experiment, which could minimize the damage on the sample.(2) In this case, we used EELS to characterize the distribution of conductive carbon, which is far more stable against the beam compared with Li2O and LiF, hence the conclusion of our work shall not be affected.Indeed, if we were studying the distribution of other beam-sensitive components, cryo-TEM with lower voltage (e.g.80 kV) might be required.
The sample was transferred in the custom-made glove box as shown below, which avoids the contamination of moist and oxygen.Relevant details have been added to the experimental section:

Manuscript:
The sample transfer from FIB to TEM was carried out in a custom-made argon-filled glove box.

Other comments:
Comment #6: No comparison with other formulation of electrodes demonstrating a worse cycling when less CA in the binder is used.The quantity of it is quite high in mass, so volume wise, there is basically acetylene black everywhere.
Response: We thank the reviewer for the insightful comments.Unlike commercially used ones, the SiOx we used in this work was not carbon-coated.Therefore, as we used a different ratio of CA (10%), the electrode could not be properly discharged or charged.We believe the total amount of carbon will not necessarily affect our conclusion as we only focus on the carbon adjacent to SiOx particles.Also, this result actually agrees with our conclusion: since there are less conductive agents around, the dilution of conductive network by SEI growth becomes more severe.

Comment #7: Washing electrodes with DMC is always a risky task: not reproducible, chemically selectiveness (soluble species) …
Response: Despite this method is a very common way to pre-treat electrode before some specific characterizations, we agree that DMC might potentially wash away some soluble species on the surface, causing slight deviation of chemical components on the surface.We hope there is better way to obtain a clean electrode without any repercussion.Nevertheless, since we have excluded detailed XPS data from the manuscript, we believe such pre-treatment will not affect the main argument considering that we focus on sub-micro scale morphologies of SEI.

Comment #8:
The embedding in a resin is a typical preparation technique for various samples (Suppl Fig 10).However, these chemical compounds are not inert ones (by definition, they need to be polymerized).Consequently, I am very doubtful on the preservation of the "real SEI" in these measurements (even when delithiated…).
Response: We thank the reviewer for the insightful comment.First, we are fully aware of the potential contamination by the resin.Therefore, we only used resin for the Nanoprobing experiment in Fig. 4a-d.Moreover, we used EpoFix epoxy resin mixed with EpoFix Hardener produced by Struers Co., Ltd. in this experiment (the detail has been added to the manuscript).
This resin was deliberately chosen due to its high viscosity, which prevented it from penetrating the dense shell of SEI.In this case the conductivity measurement of the SEI should not be an issue by the resin embedding.Response: Thank you for your comments.Indeed, the area you mentioned seems slightly dented due to the brightness and contrast.Therefore, a figure with adjusted contrast was demonstrated below for better observation.By looking closely, it could be found that the whole area rather flat, and definitely not a hole.We would also like to point out that the electronic conductivity was measured by placing two probes on the surface of selected area.If it is a hole, there would be an open circuit and we will not be able to obtain any values.Therefore, we are confident about the area we measured is SEI.

Manuscript:
…and embedded in a soft and commercial epoxy resin (EpoFix, Struers Co., Ltd.).Response: We thank the reviewer for the insightful comments.We apologize for not providing details of how we extracted CA from the EELS spectra.First, we normalized the curve based on the peak between the region of 380-420 eV, which is away from the near-edge peaks of carbon.A smaller M value represents a higher resemblance between this point, hence the chemical composition of this point in SEI is closer to the CA-binder domain.To avoid confusion, we have replaced all CA with CA-binder in the revised manuscript and calculation details are also added to the revised supplementary information: As we mentioned above, instead of existing alone, carbon black is always covered with polymeric binders, which will inevitably flatten the sharp π* peak of carbon black.Apart from that, it has been pointed out by Papworth et al., that sp2 carbon does not necessarily show a sharp π* peak, which might be due to indirect transition from 1s to π* state [Physical review B, 62.19 (2000):12628].Also, it should be noted that the signal might be affected by the adjacent polymeric binder, which will further weaken the π* peak.

Comment #11: I find the "confining" idea of the SEI not clear at all, at least in its set-up and
effectiveness: what about the pressure applied on the whole cell?What about the movement of the graphite "layer" along cycling (they could change quite substantially the electronic conductivity after moving inward within the electrode)?Furthermore, apart from the first cycle, subsequent ones have a smaller swelling ratio variation in the case of pure SiOx… Response: It is a very good question and we should have explained this better in the manuscript.
First, we would like to point out that the pressure applied on coin-cells cannot directly apply to the particles, but only to the cell shells to make sure good sealing and interfacial contact.Also, the small "spring" in coin cells can only provide a very, very limited kickback force upon electrode volume expansion.Therefore, the pressure applied on the cell could barely confine particle volume expansion.The role of the graphite coating layer is more like an electrochemically active artificial protective layer to regulate (not fully suppress) the overall electrode expansion.

Supplementary Information:
To determine the distribution of CA-binder, we normalized the curve based on the peak between the region of 380-420 eV, which is away from the near-edge peaks of carbon.Next, we used Point 0 in CA-binder domain as the reference (xi) to evaluate the resemblance between a certain point (yi) with Point 0 at the region of near-edge fine structure of carbon (280-295 eV) based on mean square error method shown as below: A smaller M value represents a higher resemblance between this point, hence the chemical composition of this point in SEI is closer to the CA-binder domain.
Next, from the SEM image shown below, it can be observed that the SiOx particles remain in the lower part of the electrode after 300 cycles.Therefore, the movement of graphite into the electrode will not be an issue since the expansion and contraction of SiOx only lead to vertical movement of graphite layer, and there is no apparent horizontal stress and deformation to cause the breakdown of the top layer.
As for the last question, please note that the swelling ratio is measured based on the thickness variation of the electrode.After experiencing the first large volume expansion-contraction, the structure of SiOx electrode became loose after the 1 st cycle, creating more rooms for free particle expansion/contraction during subsequent cycles, hence the smaller thickness variation.To avoid confusion, discussions have been added to the manuscript accordingly:

Manuscript:
Also, it should be noted that after experiencing the drastic volume expansion-contraction during the 1 st cycle, the structure of SiOx electrode became loose, creating more rooms for free particle expansion/contraction during subsequent cycles, hence the smaller thickness variation.
Since the intrinsic volume changes of bulk SiOx can be hardly confined by such a layer, it can be concluded that the coating layer has suppressed the electrode expansion by exerting pressures to ensure the close packing of SiOx particles.As a result, the free space (i.e., voids) available for SEI growth can be significantly compressed.Additionally, the graphite layer remained on the top after 300 cycles (Supplementary Fig. 14).

Supplementary Information:
Supplementary Fig. 14 | Cross-sectional SEI image of a graphite coated SiOx anode after 300 cycles and the corresponding EDS mapping of Si.It can be seen that a graphite layer layer (~5 um) is firmly covered on the top while Si remain in the lower part of the electrode.
Comment #12: When swelling is concerned in electrodes, a parameter to control is usually the porosity of the electrode.Nothing is said about that in the manuscript.
Response: Thank you for your constructive suggestions.We have measured the porosity of the electrode using N2-adsorption method.It is shown that the graphite coated electrode shows slightly higher porosity (20.3%) compared to the pristine one (17.5%),which could be due to the small gap between the coating layer and SiOx electrode.The corresponding results are added to the manuscript: Other remarks: Comment #13: The 2kV beam current was surely not 180 μA!
Response: Thank you for the notice.Sorry for the mistake.Revisions have been made to the manuscript:

Comment #14: I do not see the 1 micro Pt coating for example in Fig 2. How was it deposited?
Response: The coating can be observed in the cross-sectional image below.Pt was deposited on the electrode via a Pt gun in the FIB chamber:

Manuscript:
To prepare the TEM specimen, a layer of platinum (~500 nm thickness) was deposited to protect the first atomic layers on the surface of the SiOx particle during the FIB cut (30 kV, 0.5-3 nA).Because of the high sensitivity of lithiated materials, the electron gun was preferred to the ion gun to carry out the deposition.To minimize amorphization and damages caused to the cross section, an ionic polishing step was carried out by using an ion milling system (2 keV/30 μA).All sample preparation processes were… Response: Thank you for noticing this typo.What we meant was "commercial epoxy resin".
Response: Both 3D FIB tomography and 2D SEM characterization were carried out by FIB-SEM (FIE SCIOS-ZEISS Supra55).The detail has been added to the manuscript:

Manuscript:
The FIB-SEM samples were pre-coated with ~1 μm of platinum (Pt) via a Pt gun in the FIB-SEM chamber to prevent charging and reduce ion beam damage.
Response: Sorry about the mistake, changes have been made accordingly in the manuscript: Comment #18: The least to do is to give a value for x in SiOx.One could wonder if similar results are obtained for different values of x and it is necessary to be able to reproduce exposed results.
Response: Thank you for your kind suggestions.We have acquired a different SiOx sample.
Inductively coupled plasma optical emission spectroscopy results have shown that this sample has a much higher x value (0.98) compared with the one we used in the article (0.68).Similar SEI layer structures can be observed on this SiOx after cycling, suggesting this is a universal technique with reproducible results for different samples.Relevant contents have been added to the manuscript:

Manuscript:
For both 3D FIB tomography and 2D SEM characterization, electrodes were transferred from the glove box to the FIB-SEM (FEI Scios-ZEISS SUPRA ® 55) using a transfer vessel to avoid any exposure to air.

Manuscript:
A key finding from Fig. 2d and 2e is that…

Manuscript:
…commercial grade SiOx (x is measured be 0.68) microparticles with a D50 value of 5.2 μm... Similar SEI can be observed on SiOx particles with a much higher x value (Supplementary Fig. 9).
Elemental analysis.Different SiOx samples were dissolved in solution, then diluted for inductively coupled plasma optical emission spectroscopy (ICP-OES) measurement (Agilent ICP-OES 725ES).

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): I would like to thank the authors for their detailed responses to all the comments and questions, and the corresponding revisions/additions to the manuscript.Also, it is good to see that the authors assured us of the soundness of the proposed strategy by excluding some undesirable effects (e.g., beam damage), which makes the conclusion more convincing.Therefore, I would like to recommend the publication as is.
Reviewer #2 (Remarks to the Author): Following my comments, large changes were made to the manuscript (and supplementary).Some of them are very welcome.I must say some answers were not very clear to me.Some statements are clearly opposite to what the authors probably mean (response 1 "Admittedly, we cannot guarantee perfect sample preservation since FIB is indeed a constructive sample preparation method")!I would also advise a spelling/grammar check.
Responses # 1.I still have doubts on the FIB measurements.Without cryo conditions, Ga+ ions are too energetic for the SEI.The cryo-microtomy seems to show a similar SEI.But I doubt the images were performed at low temperature, so the electron beam could provoke the same kind of images (porous SEI) as those with FIB.I know well silicon electrodes within FIB, but maybe SiOx ones have a different behaviour.
Response #3: I beg to differ.Ar+ etching can change the interpretation of XPS spectra.On survey spectra, the effect cannot be seen so Fig. R3 is not very informative.
Response #6: as expected, with less CA the electrode performance is much worse.The authors state that it basically does not matter and even does agree with their conclusions.I doubt that.But I agree this is not the main result of the study.
Response #13: to me it is still not clear.What is the "ion milling system"?Is that an ion polisher using Argon ions?Or is it still the FIB??This must be clear since sample preparation is at the core of the publication.
Response #14: it is rather surprising that the authors indicate that they deposited Pt all around the SiOx particles (see Fig; R7a).
Response #16: to be clearer, FIE does not exist (must be FEI… ) and SCIOS is a FIB from FEI not ZEISS.
So please write "FEI SCIOS and ZEISS supra 55".SCIOS for 3D and Supra for 2D?

General comment
The authors present a study of SEI growth on SiOx electrodes through FIB-3D visualization.They proposed a model for the SEI growth based on formation on porous SEI layer underneath a "dense SEI" during cycling.At the end of the paper, they proposed to "confine" the electrode using a graphite on top of it to restrict the SEI formation which seems to have a positive effect.I am not fully convinced by the mechanism proposed and some of the results are questionable.
The morphology of the SEI (sample after 300 cycles) after ion milling cross sectioning and cryoultramicrotomy (Fig S10) is said to be the same and that beam damage are negligible.I do not fully agree with this answer.A porosity is indeed visible on sample after cryo-ultramicrotomy however it seems less pronounced than the one visible on ion milled sample.As the SEI is most probably composed of a mixture of polymer-like compounds as well as inorganic ones, a differential pulverization between the different compounds in the SEI and even the SiOx is highly possible.One can then wonder about an effect of the Ga+ irradiation on an increase in porosity or even generation of porosity (through gas generation for example which is a classical observation on polymer-like compounds).
The proposed mechanism is surprising in the sense that the "dense" layer which is progressively formed during cycling, does not crack or break while a porous part often linked to a volume expansion grows underneath.How exactly is the volume expansion due to the formation of an underlying porous structure accommodated without breaking the upper layer as generally observed in silicon-based anode?
On the EELS results, I am not fully convinced by the mapping allowing the distribution of the carbon from CA-binder and the rest of the inorganic or organic polymer-like compounds.The reference for the CA-binder was taken in a sample already submitted to cycling and to milling so I disagree about taking it as reference for a mapping.The carbon present in the polymer-like compounds in the SEI can also have a similar shape.The analyses of a real reference of CA-binder taken on a fresh sample would have been preferable and the investigation of the valence (plasmon) electron energy loss spectroscopy (VEELS) which can also be used as a fingerprint would have been interesting.
In Figure S8, at the end of the caption, it is referred to Figure S10 "d-e" but there is no "e" in figure S10.
In figure S10, "b" and "d" are an enlarge view of respectively "a" and "c", it should be precise in the caption.
In figure S14, it is written "the corresponding EDS mapping of Si and C." but the carbon map is not presented.
In my opinion, the author should put the non-etched results as the observations in the global spectra between non-etched and etched samples are really similar.The conclusion will remain the same.However, an XPS spectrum is really dependent of the analyzed area so I would be more moderate about the carbon conclusion.
Comment #5: Response #13: to me it is still not clear.What is the "ion milling system"?Is that an ion polisher using Argon ions?Or is it still the FIB??This must be clear since sample preparation is at the core of the publication.

Response:
We thank the reviewer for the detailed comments.Yes, it is an ion polisher using Ar ions (as shown in Fig. R1).The relevant details have been included in the revised manuscript: Main text:

Methods
To minimize amorphization and damages caused to the cross section, an ionic polishing step was carried out using an Ar-ion polishing system (Gatan Precision Ion Polishing System, PIPS II Model 695) under 2 keV/30 μA.

Reviewer #3
General Comments: The authors present a study of SEI growth on SiOx electrodes through FIB-3D visualization.They proposed a model for the SEI growth based on formation on porous SEI layer underneath a "dense SEI" during cycling.At the end of the paper, they proposed to "confine" the electrode using a graphite layer on top of it to restrict the SEI formation which seems to have a positive effect.I am not fully convinced by the mechanism proposed and some of the results are questionable.
Response: We are grateful for the time and effort taken in reviewing our manuscript.Revisions have been made accordingly to the reviewer's constructive comments that would significantly improve this work.
Comment #1: The morphology of the SEI (sample after 300 cycles) after ion milling cross sectioning and cryo-ultramicrotomy (Fig S10) is said to be the same and that beam damage are negligible.I do not fully agree with this answer.A porosity is indeed visible on sample after cryoultramicrotomy however it seems less pronounced than the one visible on ion milled sample.As the SEI is most probably composed of a mixture of polymer-like compounds as well as inorganic ones, a differential pulverization between the different compounds in the SEI and even the SiOx is highly possible.One can then wonder about an effect of the Ga + irradiation on an increase in porosity or even generation of porosity (through gas generation for example which is a classical observation on polymer-like compounds).
Response: We would like to thank the reviewer for the insightful comments.After careful deliberation, we agree with the reviewer that the beam damage to the SEI, especially the organic components, is not negligible.We apologize for the unsolid claims, and the corresponding revisions have been made to the manuscript:

Main text:
Additionally, the interference of beam damage to the sample was studied by comparing samples prepared from FIB and cryo-ultramicrotomy (Supplementary Fig. 10).Although a slightly higher porosity can be observed in the sample prepared by FIB (which can be attributed to the possible beam damage), both samples exhibit SEIs with a loose interior layer and a dense exterior layer, confirming the suitability of this method for observing SEI with considerable thickness (sub-micron level).In comparison, for anode materials with thin and fragile SEI layers (e.g.Si), both Ga + and electron beams might distort the SEI morphology.
Nevertheless, it should be noted that the sample prepared through cryo-ultramicrotomy still shows porous SEI structure in the inner layer, despite the pores are less pronounced than the sample prepared from FIB, and both samples exhibit dense outer SEI layers.This result could be attributed to that the loose inner SEI layer is more susceptible to Ga + beam, hence the slight porosity increase.
On the whole, the difference in porosity will not necessarily affect our main conclusion, where two types of SEI are formed on SiOx -the dense inner layer and the loose inner layer.
More importantly, the reviewer's comments have also brought us a new revelation that since the porosity could be affected by sample preparation methods, it is no longer appropriate to define Type I SEI as "porous".Instead, we believe "loose" and "incompact" are more suitable to describe Type I SEI.Some representative changes in both texts and figures have been made as shown below.
We hope our revisions could relieve the reviewer's concerns.

Main text:
…we reveal an exceptionally characteristic SEI microstructure with an incompact inner region and a dense outer region… …there appear to be a boundary developed on the surface of this outer layer and the originally loose space between SiOx particles is filled out… As the cycle continues, the loosely structured Type I SEI not only grows thicker, but also evolves into the dense Type II SEI… Comment #2: The proposed mechanism is surprising in the sense that the "dense" layer which is progressively formed during cycling, does not crack or break while a porous part often linked to a volume expansion grows underneath.How exactly is the volume expansion due to the formation of an underlying porous structure accommodated without breaking the upper layer as generally observed in silicon-based anode?
Response: We thank the reviewer for the insightful comments, which are very helpful for improving this manuscript.
First, it should be noted that being dense is not necessarily equivalent to being rigid.We apologize for using the term "hard shell" to describe Type II SEI, which might cause the confusion.Since SEI layers generally consists both organic and inorganic components, the former facilitates reasonable resilience to tolerate the small stress caused by the volume change.Therefore, a thin dense layer could progressively grow into a thick dense layer during long-term cycling.
Corresponding changes have been made to avoid further misunderstanding:

Main text:
As the cycle continues, the loosely structured Type I SEI not only grows thicker, but also evolves into the dense Type II SEI, whose morphology remains relatively stable during sequential cycles.
Upon delithiation, the Type I SEI is stretched with the shrinkage of SiOx without causing structural collapse of Type II SEI; whereas during lithiation, the Type I SEI is compressed between the "dense shell" (Type II SEI) and the expanding SiOx particle, blurring the boundary between Type I and Type II regions.
It should be noted that due to the existence of soft polymeric SEI components, the outer layer can mechanically withstand the volume changes as most of the stress can be buffered by the inner layer.
Second, by looking at just one lithiation-delithiation cycle, the delithiated state is illustrated in Fig. R1, where compressed Type I and Type II SEI co-existed.To avoid further confusion, we have edited Fig. 3 and the corresponding discussions as below: Main text: Thirdly, we understand that for Si anodes, such SEI thickening has not been observed.This is due to the large volume change of Si (~300%) does not allow such process: on the one hand, nanosized Si cannot support such thick SEI; on the other hand, for micro-sized Si particles, even they can avoid pulverization (which is always the case), we speculate that most Type II SEI (e.g.without electrolyte optimization or material modification) cannot be formed as the large volume swing will destroy it at the very beginning.In comparison, exhibiting a moderate volume swing, the surface of SiOx is suitable for SEI thickening.To better demonstrate significance of this work, we have included the following discussions in the revised manuscript:

Main text:
Interestingly, SEI with such thickness has not been observed on Si anodes, which might be attributed to the large volume variation of Si generally leads to particle pulverization and collapse of SEI during its early growing stage, hence no Type II SEI can be formed.
Comment #3: On the EELS results, I am not fully convinced by the mapping allowing the distribution of the carbon from CA-binder and the rest of the inorganic or organic polymer-like compounds.The reference for the CA-binder was taken in a sample already submitted to cycling and to FIB milling so I disagree about taking it as reference for a mapping.The carbon present in the polymer-like compounds in the SEI can also have a similar shape.The analyses of a real reference of CA-binder taken on a fresh sample would have been preferable and the investigation of the valence (plasmon) electron energy loss spectroscopy (VEELS) which can also be used as a fingerprint would have been interesting.

Response:
We are truly thankful for the reviewer's constructive comments.We agree that the reference should comes from a fresh sample.Therefore, we have taken a new reference from a pristine SiOx electrode and re-drawn EELS mapping based on the new reference, which does looks a bit different.The corresponding revisions are listed below:

Main text:
Moreau and co-workers have also employed low-loss EELS to identify SEI species on nanosized Si anode. 22  To directly prove this assumption, EELS was used to analyze different areas of an electron transparent FIB lift-out lamellae including a SiOx particle, its SEI and surrounding CA-binder matrix (Fig. 4e).The reference point was sampled from the CA-binder domain in a pristine SiOx electrode, which exhibits very similar C K-edge spectra with CA-binder domain in the cycled electrode (Supplementary Fig. 13).
Next, we quantitatively determine the degree of similarity between signals collected from a selected domain of SEI on a cycled SiOx (Fig. 4g) with the reference.Consequently, an EELS mapping (Fig. 4h) of the relative conductive composition concentration can be obtained (see detailed calculation descriptions in Supplementary Fig. 13).
Fig. 4h | The planar concentration distribution of conductive agents.
complicated to extract the desirable information of CA-binder mixture.Still, we thank the reviewer for broadening our vision, which is very useful in our future componential study of SEI.

Fig. R3 |
Fig. R3 | Comparison between XPS results of the SiOx anode a-e, after and f-j, before Ar + etching.

Comment # 9 :
In Suppl.Fig 12, the "SEI" part looks more like a hole due to beam damage than a solid (porous) phase.Hence, I do not find the electric conductivity measurement on this part convincing enough.

Fig. R5 |
Fig. R5 | Comparison between the SEM image in Supplementary Fig. 12 (origin submission) before (left) and after adjusting brightness and contrast (right).
Fig. R6 | Examples of how the mean square error was calculated.yi and xi are normalized value of the sample to be measured and Point 0, respectively.

Fig. R7
Fig. R7 | a, Cross-sectional SEM image of a cycled SiOx electrode after Pt coating.b, SEM image of a Pt gun above the substrate.

Fig. R2 |
Fig. R2 | Deformation process of Type I SEI during cycling.

Fig. R3 |
Fig. R3 | Electron transparent FIB lift-out lamellae of a, cycled and b, pristine SiOx particle and its SEI with surrounding CA-binder domain.c, low loss spectra of different points in different electrodes.